Location determination systems are used in the prior art to identify one or more locations where a given wireless terminal may be found when responding to an emergency (911) call, or in tracking a suspect or a missing person, etc. It is well known in the art that the signal strength of a wireless signal received from a transmitter differs among locations, and, therefore, the location of a wireless terminal can be estimated by comparing the signal strength it currently experiences against a map or database that correlates locations to signal strengths. For example, if a particular radio station is known to transmit a strong signal to a first location and a weak signal to a second location, and a given wireless terminal at an unknown location is receiving the radio station with a weak signal, it is more likely that the wireless terminal is at the second location than it is at the first location.
In regard to wireless networks, a wireless terminal generally receives wireless signals from several antennas in the network. The received signals vary in signal strength with respect to each antenna, and vary further as the wireless terminal moves from location to location. This variability provides a kind of pattern that is indicative of the location where the wireless terminal received the signals. A simplified example illustrates this point. A first antenna, Antenna A, transmits a strong signal to Location 1 and Location 2, but a weak signal to Location 3, Location 4, and Location 5; and a second antenna, Antenna B, transmits a strong signal to Location 1, Location 3 and Location 5, but a weak signal to Location 2 and Location 4. This information is summarized in Table 1 below and forms the basis for a map or database that correlates locations to signal strength.
TABLE 1Illustrative Signal Strength Database (Absolute Signal Strength)Antenna AAntenna BLocation 1Strong SignalStrong SignalLocation 2Strong SignalWeak SignalLocation 3Weak SignalStrong SignalLocation 4Weak SignalWeak SignalLocation 5Weak SignalStrong SignalIf a given wireless terminal at an unknown location receives Antenna A with a weak signal and Antenna B with a strong signal, it is more likely that the wireless terminal is at Location 3 or Location 5 than at any of Locations 1, 2, or 4.
The location analysis does not necessarily end here. After Location 3 and Location 5 have been identified as candidate locations, further analysis is required to resolve—if possible—which of the two locations to report as the estimated location of the wireless terminal. It is important to report speedily and accurately, especially when time is of the essence, for instance in responding to an emergency (911) call, in tracking a suspected criminal/terrorist, in tracking a missing person, etc. However, the prior art techniques are insufficient to help choose between Location 3 and Location 5. A new approach is needed.